Hitherto, all-solid-state lithium secondary batteries including solid electrolytes and active materials have been reported. Such a lithium secondary battery has a problem with lithium-ion conductivity at the solid-solid interface between a solid electrolyte and an active material, in some cases. For example, in the case where the solid electrolyte and the active material are bound together by the application of external pressure, a strong adhesion of bonded surfaces is not obtained, in some cases. In the case where sintering is performed at a high sintering temperature to increase the adhesion of an interface, the high temperature can lead to the degradation of the solid electrolyte and the active material and can lead to the formation of a third phase that increases the interface resistance between the solid electrolyte and the active material. Thus, for example, the lithium-ion conductivity between the solid electrolyte and the active material is inhibited, so that a sufficient energy density is not provided, in some cases.
To address the foregoing problems, for example, a technique has been reported as follows: An active material containing a first crystalline substance capable of intercalating and deintercalating lithium ions is bound to a solid electrolyte containing a second crystalline substance having lithium-ion conductivity. When analysis is performed by X-ray diffractometry, a component other than constituents of the active material layer and constituents of the solid electrolyte layer is not detected (for example, see PTL 1). A technique in which a sol-gel method is employed for the formation of an active material on a surface of a solid electrolyte has also been reported (see NPL 1). Furthermore, a technique in which a pulsed laser deposition (PLD) method, which is a gas-phase method, is employed has been reported (see PTL 2).